_sass: Move abstract-card mixin into card.scss

_sass/custom/card.scss

+@mixin abstract-card() {
+  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.07), 0 4px 14px rgba(0, 0, 0, 0.05);
+  margin: $sp-4 -$gutter-spacing-sm;
+
+  @include mq(md) {
+    border-radius: $border-radius;
+    margin: $sp-4 0;
+  }
+}
+
 %card {
   @include abstract-card();
   display: flex;

_sass/custom/custom.scss

 // Just the Class dependencies
-@import 'mixins';
 @import 'card';
 
 // Just the Class styles

_sass/custom/mixins.scss

-@mixin abstract-card() {
-  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.07), 0 4px 14px rgba(0, 0, 0, 0.05);
-  margin: $sp-4 -$gutter-spacing-sm;
-
-  @include mq(md) {
-    border-radius: $border-radius;
-    margin: $sp-4 0;
-  }
-}